Flow-forming is a known metal-forming technique in which a piece of material to be worked is secured to a mandrel and rotated while one or more rollers are used to apply pressure to an external surface of the work piece in order to deform the work piece as it is rotated. The roller compresses the work piece against the mandrel causing the work piece to become deformed both by lengthening it axially and thinning it radially. Owing to the large amount of pressure applied by the roller or rollers to the work piece, and owing to the rotation of the work piece relative to the roller or rollers, the work piece is subjected to a large amount of friction during the flow forming process. Consequently, the temperature of the work piece can increase to several hundred degrees Celsius during the process. Such a high temperature can have an undesired effect on the work piece, such as changing the properties of the material from which the work piece is formed. Furthermore, the rollers can become worn, and regularly need replacing. Regular replacement of rollers can lead to high running costs.